Re: [PATCH 1/2] arm64: cpufeature: Add GIC CPUIF v4.1 detection

[Marc Zyngier]

On 2020-11-11 16:28, Lorenzo Pieralisi wrote:
GIC v4.1 introduced changes to the GIC CPU interface; systems that
integrate CPUs that do not support GIC v4.1 features (as reported in 
the
ID_AA64PFR0_EL1.GIC bitfield) and a GIC v4.1 controller must disable in
software virtual SGIs support since the CPUIF and GIC controller 
version
mismatch results in CONSTRAINED UNPREDICTABLE behaviour at 
architectural
level.
Add a cpufeature and related capability to detect GIC v4.1 CPUIF
features so that the GIC driver can probe it to detect GIC CPUIF
hardware configuration and take action accordingly.

Signed-off-by: Lorenzo Pieralisi <lorenzo.pieral...@arm.com>
Cc: Will Deacon <w...@kernel.org>
Cc: Catalin Marinas <catalin.mari...@arm.com>
Cc: Marc Zyngier <m...@kernel.org>
---
 arch/arm64/include/asm/cpucaps.h |  3 ++-
 arch/arm64/kernel/cpufeature.c   | 10 ++++++++++
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/cpucaps.h 
b/arch/arm64/include/asm/cpucaps.h
index 42868dbd29fd..35ef0319f422 100644
--- a/arch/arm64/include/asm/cpucaps.h
+++ b/arch/arm64/include/asm/cpucaps.h
@@ -65,7 +65,8 @@
 #define ARM64_HAS_ARMv8_4_TTL                  55
 #define ARM64_HAS_TLB_RANGE                    56
 #define ARM64_MTE                              57
+#define ARM64_HAS_GIC_CPUIF_VSGI               58

-#define ARM64_NCAPS                            58
+#define ARM64_NCAPS                            59

 #endif /* __ASM_CPUCAPS_H */
diff --git a/arch/arm64/kernel/cpufeature.c 
b/arch/arm64/kernel/cpufeature.c
index dcc165b3fc04..9eabbaddfe5e 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -2136,6 +2136,16 @@ static const struct arm64_cpu_capabilities
arm64_features[] = {
                .cpu_enable = cpu_enable_mte,
        },
 #endif /* CONFIG_ARM64_MTE */
+       {
+               .desc = "GIC CPUIF virtual SGI",

nit: that's not really what this feature is. It only means that the
sysreg interface complies to v4.1. Which on its own is totally rubbish,
because the sysreg don't change behaviour between 3.0/4.0 and 4.1.

+               .capability = ARM64_HAS_GIC_CPUIF_VSGI,
+               .type = ARM64_CPUCAP_BOOT_CPU_FEATURE,
+               .matches = has_cpuid_feature,
+               .sys_reg = SYS_ID_AA64PFR0_EL1,
+               .field_pos = ID_AA64PFR0_GIC_SHIFT,
+               .sign = FTR_UNSIGNED,
+               .min_field_value = 3,
+       },

Do we really need a new cap for that? Or can we rely on simply looking
at the sanitised feature set? I'm not overly keen on advertising a 
feature
at CPU boot time if we discover later on that we cannot use it because 
all
we have in a non-4.1 GIC.

Another thing is that we currently assume that *all* CPUs will be the 
same
at the point where we setup the GIC (we only have a single CPU booted at 
that
point).

        M.
--
Jazz is not dead. It just smells funny...